Two known Bi-Pb-Sr-Ca-Cu-O superconducting phases have been reported with zero resistance temperatures at 80K and 110K [Maeda et al., Jpn. J. Appl. Phys. 27, L-209 (1988)]. The 80K phase has a limited application interest though prepared easily. To purify 110K phase needs more care, but it is conceivably applicable due to the high Tc above liquid nitrogen temperature (77K).
Prior techniques for preparing Bi-based superconductors include (1) solid state reaction, (2) co-precipitation and (3) sol-gel. The disadvantages of these methods are described as follows:
(1) Solid state reactions mix metal oxides or metal carbonates by manual grinding followed by heat treatment. The method requires extremely high temperature and long heating period, which result in powders with poor morphologies (low surface area, less homogeneous, etc.).
(2) Co-precipitation method has been utilized in simple ceramics. For Bi-based superconductors with multiple cations, the technique has difficulty in obtaining stoichiometric precipitates because of the solubility constant (Ksp) differences. To adjust pH value by alkali metal hydroxide (NaOH or KOH) may contaminate the solution with metal remnants. Japan Kokai JP 1197252 disclosed a method for obtaining 55K superconductors using ammonia or triethylamine for pH adjustment, which was not considered successful.
(3) Sol-gel method provides a relatively low temperature chemistry route using an organic solvent to emulsify different solid particles into semirigid gels. The common candidates for organic solvent are acetic acid, oxalic acid and citric acid. Organic solvent is a chelating agent for metal cations and will be removed by extraction or evaporation. After thermal decomposition, the dehydrated gels lead to the final produces. U-Ba-Cu-O superconductor has been prepared by the sol-gel method. However, it was considered impractical for Bi-Pb-Sr-Ca-Cu-O system because of the low solubility of bismuth salt. Japan Kokai JP 1224262 tried to dissolve bismuth salt in strong acid. Japan Kokai JP 1219004 used EDTA and ethylene glycol for chelating agents. Unfortunately, none of these prior Patents could produce pure 110K superconducting phase.
Pure 110K superconducting phase is essential in the application, such as the wire drawing and the film preparation. Several early reports experienced a zero resistance at 110K, but XRD and magnetization measurements showed only a minor amount of 110K phase [Mizuno et al., Jpn. J. Appl. Phys. 27, L-1225 (1988), Aota et al., Jpn. J. Appl. Phys. 28, L-2196 (1989), Hakuraku et al., Jpn. J. Appl. Phys. 28, L-67 (1989)]. Experimentally, we have learned that zero resistance could be achieved at 110K if there is a conducting network formed by 80K phase plus some 110K grains. In order to guarantee the physical properties after manufacturing, a high purity 110K phase must be synthesized.